There are two different settings for room invite permissions - Administrator Only and All Members.
The default is Administrator only, and as the name implies, only a room admin can invite other users to the room. If set to All Members, anyone can add a new member to a room.
To change the invite permissions, tap the ellipses (Mobile) or click the gear (Desktop) to bring up the room controls.
Select Edit Room, then under "Who can invite new members to the room, click or tap your selection.